It is known that an oxygen smelting converter is a reactor which enables, in principle, the removal down to a very low content of the oxidizable elements of the pig iron, the principal ones being carbon, phosphorus and silicon. However, despite significant progress (blasting of oxygen from the bottom, or mixed blasting, from the bottom and from the top), it still seems difficult to obtain results which are well reproducible from one installation to another.
Moreover, the trend in the marketplace toward steel of an increasingly greater degree of purity implies a substantial increase in the manufacturing cost for the details provided on the converter, or even, sometimes, an impossibility of providing for a single converter all the required specifications.
It is known that one response to these problems consists in separating in both time and space the diverse oxidation operations which enable passing from pig iron to steel (desilication, dephosphorization, decarburization), so as to optimize them separately.
The desilication and dephosphorization operations conducted under these conditions characterize what will henceforth be called the pre-treatment, or more precisely the pre-smelting of the pig iron.
The known pre-smelting installations all proceed by sequential operations (desilication, then dephosphorization, then decarburization) on discrete and repetitive quantities of pig iron (see, e.g., British Specification No. 2,072,221).
One part of the liquid pig iron mass to be pre-treated is in effect drawn off successively into several independent metallurgical receptacles, each more or less specialized for the treatment phase to which it is assigned, or placed in a single receptacle, generally a straight ladle, which is brought adjacent to fixed locations for successive treatments distributed in the workship. (French Patent Appln. No. 2,439,821 of NSC).
Whatever their type, these installations still do not give complete satisfication. In particular, the transfers of the pig iron into different receptacles, or from one treatment location to the next, give rise to waiting times, therefore also to thermal losses, and quite often even to a deterioration of the metal yield by reason of intermediate scraping operations for purifying the slag formed in each pre-smelting operating phase. In addition, since the operation is discontinuous, each of these phases is cyclical and therefore involves transitory operating periods which are poorly adapted for control, and which diminish the effectiveness of the entire treatment, and this even on the assumption that the sequence of successive phases can be correctly time-coordinated.